The present invention relates to a structure of an acoustic surface wave filter bank.
An acoustic surface wave filter bank consists of a single or a plurality of piezoelectric substrates on which a plurality of acoustic surface wave filters each comprising input and output interdigital electrodes are fabricated. As the size of the filter bank is small, the application field of a filter bank has recently grown, and the requirement for the miniaturization and the improvement of electrical performance has been increased.
Conventionally, an acoustic surface wave filter bank consisting of a plurality of acoustic surface wave filters with different center frequencies allocates the filters in a package in order of center frequency.
FIG. 1 shows a prior arrangement of filters, in which five filters F.sub.1, F.sub.2, F.sub.3, F.sub.4 and F.sub.5 are included. In the figure, the numerals 1, 2, 3, 4 and 5 are input transducers of the filters F.sub.1, F.sub.2, F.sub.3, F.sub.4 and F.sub.5, respectively, the numerals 6, 7, 8, 9 and 10 are output transducers of the filters F.sub.1, F.sub.2, F.sub.3, F.sub.4 and F.sub.5, respectively, and 11 is a piezoelectric substrate. FIG. 1A is the embodiment where five filters are fabricated on one substrate, FIG. 1B is the embodiment where two substrates are used so that first group of filters F.sub.1, F.sub.2 and F.sub.3 are fabricated on a first substrate and second group of filters F.sub.4 and F.sub.5 are fabricated on a second substrate. FIG. 1C is the embodiment where each filter F.sub.1 through F.sub.5 is fabricated on a related individual substrate.
When an acoustic surface wave filter bank is used as a frequency de-multiplexer or a frequency multiplexer, the input transducers 1, 2, 3, 4 and 5 are interconnected considering impedance matching.
FIG. 2 shows ideal frequency response of the acoustic surface wave filter bank in which the horizontal axis shows frequency and the vertical axis shows amplitude. In the figure, the numerals 12, 13, 14, 15 and 16 are amplitude responses of the filters F.sub.1, F.sub.2, F.sub.3, F.sub.4 and F.sub.5 each of which has the center frequency f.sub.1, f.sub.2, f.sub.3, f.sub.4 and F.sub.5, respectively. FIG. 2 does not show sidelobes for the sake of simplicity of the drawing. FIG. 3 shows detailed amplitude response to the filter F.sub.3 with sidelobes.
However, a prior acoustic surface wave filter bank has the disadvantage that the filter response of each filter is affected by the adjacently located filters, because the center frequency of each filter is very close to the center frequency of the adjacently located filters. As a result, ripples in amplitude and phase appear in pass bands of filters, and the attenuation level in the out of band region becomes insufficient. Thus, the response of a filter bank becomes unsatisfactory.
The above problem is described in detail in accordance with FIG. 4, which shows the propagation of acoustic surface waves in the filter bank.
In FIG. 4, the solid lines show desired direction of propagation, and the dotted lines show undesired direction of propagation from adjacently located filters. For instance, the output transducer 8 of the filter F.sub.3 receives not only the desired acoustic surface wave generated by the input transducer 3, but also the undesired leakage waves which are generated by the input transducers 2 and 4 of the adjacently located filters F.sub.2 and F.sub.4. Of course, there exist other leakage waves from input transducers of the other filters which are not adjacently located. However, the level of those leakage waves are so small that those waves are not shown in FIG. 4.
It should be noted that the filter characteristics of an acoustic surface wave filter is the product of the filter characteristics of an input transducer and the filter characteristics of an output transducer. Hence, the frequencies of the desired wave and the leaked waves become adjacent to each other, if the center frequencies of the adjacently located filters are adjacent to each other as in the conventional filter bank. For example, the output transducer 8 of the filter F.sub.3 in FIG. 4 receives three acoustic surface waves with frequency responses shown in FIG. 5. In FIG. 5, curve 17 shows the frequency response of the acoustic surface wave generated by the input transducer 3 of the filter F.sub.3, and curves 18 and 19 show the frequency responses of the acoustic surface waves which are generated by the input transducers 2, 4 of the filters F.sub.2, F.sub.4 and leaked to the output transducer 8 of the filter F.sub.3.
These three acoustic surface waves received by the output transducer 8 are further shaped by the filter characteristics of the output transducer 8. For instance, when three acoustic surface waves shown in FIG. 5 are received by the output transducer 8 of the filter F.sub.3, the final characteristics of the filter become as shown in FIG. 6. In comparing the ideal characteristics shown in FIG. 3 with the actual characteristics in FIG. 6, it should be noted that the curve of FIG. 6 has a ripple in the pass band because of the undesired leakage, and further the attenuation in the out of band region is not sufficient, therefore, the characteristics of the filter bank are considerably distorted.
The distortion of the filter characteristics of an acoustic surface wave filter bank is in particular large when the filters are located in a package very close to each other, which restricts the miniaturization and the performance improvement of an acoustic surface wave filter bank.